The mineral deposits in the Guanajuato area are classic fissure-hosted lowsulphidation epithermal gold-silver-bearing quartz veins and stockworks.

At the Guanajuato Mine, mineralization occurs within fault zones along the Veta Madre vein system as discontinuous shoots and tabular bodies. It is apparent from mine plans that stopes can be in the order of 700m long and extend for 400m vertically. Zone thickness ranges from centimetrescale to tens of metres. A series of mineralizing events is thought to have taken place during the Oligocene, a period of intense felsic volcanic activity in the area, and comprised three stages termed pre-ore, ore, and post-ore. Pre-ore mineralization consists of trace silver and gold with accessory quartz and adularia. Ore mineralization comprises an early silver-rich phase associated with adularia, as well as a later low-silver variant, which is typified by calcite and quartz. The postore mineralization is also precious metal-poor, with accessory calcite, dolomite, and fluorite.

The primary economic components are silver and gold, with silver the more important of the two. Base metals do not normally occur in economic concentrations. Average silver grades of the ore are typically in the 100g/t Ag to 500g/t Ag range but locally can be over 1,000g/t Ag. Gold grades are generally in the 0.5g/t Au to 2g/t Au range, with the exception of the Santa Margarita vein where average grades are in the range of 5g/t Au to 7g/t Au. Relative gold and silver contents at Santa Margarita are quite different from Cata, Pozos and Guanajuatito. The average silver to gold ratio in Cata is roughly 225:1, at Pozos 250:1, at Guanajuatito 275:1 while at Santa Margarita, it is 3.5:1. Within the mine, drill core and channel samples are not normally analysed for base metals so average grades for Cu, Pb or Zn have not been obtained.

Mineralization at Guanajuato is closely associated with the structural history. The “Veta Madre” quartz-adularia vein / breccia system is closely associated with the Madre fault and an associated diorite dyke (thickness varying from discontinuous lenses at Guanajuatito to a 50-100m thick body in the Cata, Los Pozos, and Santa Margarita areas), oriented 325 degree azimuth with a 45 degree southwest dip. The Veta Madre forms along the dyke contacts, and in the Esperanza Formation, the footwall rocks to the Madre fault. At the Guanajuatito zone the main mineralization occurs just into the deformed siltstone and shale of the Esperanza Formation. Four zones were modeled at Guanajuatito, with the Veta Madre and the closely associated footwall (FW) zone being dominant below the 80 level. At the Cata zone, Veta Madre mineralization occurs along the base of the diorite dyke with the Esperanza Formation, and as seven separately modelled zones within the diorite. A number of these zones are shallow dipping structural splays. The Los Pozos and Los Pozos SE zones are vein stockwork to breccia systems (Veta Madre) at the base of the diorite dyke and into the Esperanza Formation. The Santa Margarita zones form a complex structural set of four bodies within the diorite dyke and at its upper contact with the Guanajuato Formation conglomerates or basal andesite. These are above the Veta Madre breccia which is at the diorite contact with the footwall Esperanza Formation, but in this area is barren. The San Cayetano zone occurs deep in the Veta Madre south of the Rayas shaft, and tends to be narrow and often in the upper portion of the Veta Madre. The Promontorio zone occurs in the hanging-wall Guanajuato Formation conglomerates immediately above the Veta Madre structure at the contact of the Guanajuato Formation and the diorite dyke. At Valenciana there are parallel mineralized structures (Veta Madre) at the Esperanza Formation – diorite contact and into the Esperanza Formation.

The best mineralization is often found related to bends in the Veta Madre orientation such as at San Vicente in the Rayas area, and at Cata and Santa Margarita. These structural bends may be due to changes in rock type competencies, and varying thickness of the diorite dyke.

The vertical extent of the deposits at Guanajuato spans over 700m (2,200m to 1,500m elevations and open to depth). Mineralization occurring above 2,100m elevation was termed “upper ore”, between 2,100m and 1,700m “lower ore”, and below the 1,700m elevation “deep ore”. Fluid inclusion microscope work from over 850 samples gather through the mine and in deep drilling from the Santa Margarita area, indicated boiling zones from the 2,100m to 1500m (deepest drilling at the GMC) elevations. Structural observations of up to eight stages of crosscutting brecciation, and the variable range of Ag:Au ratios indicate that the mineralization along the Veta Madre is associated with multi-phase structural activity and fluid flow.

Mining of Cata Clavo has reached the 540 level where development and stoping are being carried out on the Veta Madre vein. Cata Clavo is a relatively steeply dipping structure that has been identified for mining from the 1560 elevation (540 level) to the 1665 elevation (435 level). The mining area is up to 100 metres long and up to 10 metres wide. The Veta Madre FW, Contact, Veta Madre HW, Alto 1 to Alto 4 zones are located on the hanging wall side of the Veta Madre and the multiple zones are close to one another. In some areas, the combined Veta Madre to Alto 4 generates an overall width exceeding 30 metres.

The Cata ore zones are being mined by a mechanized cut and fill method. An access ramp is located in the hanging-wall from which cross-cuts are driven to access the ore at the various lift elevations. The ramp is approximately 3.5 metres by 3.5 metres and is supported by a combination of grouted rebar, wire mesh and straps as required. There are as many as six separate ore zones varying in widths from two to eight metres with strike lengths of 20 to 80 metres.

There are 1-boom, electric hydraulic jumbos for drilling, 3.5 and 4.0-yard LHDs for mucking, and 10 and 18-tonne trucks for ore haulage. Ground support in the stopes is installed using scissor lift trucks.

At the Los Pozos SE zone, mining is underway above the 275 level, both on remnant mineralization as well as recovery of old pillars. Below the 275 level, the zone is in-situ. The Los Pozos deposits are relatively steeply dipping structures that have been identified for mining from the 1825MASL elevation (275 level for Pozos NW & 250 level for Pozos SE) to the 1690MASL elevation (410 level).

Los Pozos ore zones are being mined by a mechanized cut and fill method. There is a ramp in the footwall which provides access to sublevels that are spaced approximately 35 metres apart.

The ramp is approximately 3.5 metres by 3.5 metres and is supported by grouted rebar and mesh as equired. At each sublevel, a crosscut is driven across the zone followed by development along strike to the extents of the vein. The vein is then opened to its full width or, if the zone is too wide, to a maximum of 12 metres wide. In wider zones, it is necessary to either install cable bolts and/or to leave pillars.

Mining then progresses upwards in three metre lifts using breasting to mine the ore. As ore is removed, waste fill is placed in the void. There are electric hydraulic jumbos for drilling, 3.5 and 4.0-yard LHDs for mucking, and 10t trucks for ore haulage. Ground support in the stopes is installed using scissor lift trucks.

For five metre widths at Los Pozos, only rock bolt support is required. For widths over five metres but less than 12 metres, cable bolts are required for support. Four-metre-long cable bolts are installed in holes drilled with the jumbo. For zones over 12 metres wide, it is necessary to leave a permanent pillar or to provide support which is longer than the four-metre cable bolts.

The Santa Margarita gold-rich vein has been explored by ramp development from the 390 level to the 500 level. The ramp has been driven from the 1710MASL elevation (390 level) and extended down to the 1600MASL elevation (500 level). Extractions are completed by cut and fill mining methods. In 2014, there was re-habilitation and development on the 345 and 360 levels.

Santa Margarita is a gold rich deposit that is moderately dipping and narrow. Development of Santa Margarita has consisted of an exploration development driven on the zone, with ramp access driven in the footwall rocks.

The Santa Margarita ore zones are being mined by a mechanized cut and fill method. An access ramp is located in the hanging-wall from which cross-cuts are driven to access the ore at the various lift elevations.

The ramp is approximately 3.5 metres by 3.5 metres and is supported by grouted rebar, as required. There is one main breccia ore zone plus vein stockwork ore zones located in the footwall. Typical ore widths are 2.0 to 5.0 metres. The main zone is over 150 metres long while the footwall stockwork zones are 20 to 50 metres long.

Drilling is carried out with hand-held jack-leg drills in the narrow sections and with 1-boom, electric hydraulic jumbos in the wider zones. Mucking is by 2.0, 3.5 and 4.0-yard LHDs and broken ore is trucked to the Cata shaft by 10 and 18-tonne trucks. Ground support in the stopes is installed using stoppers or jack-legs and / or scissor lift trucks.

The Guanajuatito ore zones are being mined by a mechanized cut and fill method. An access ramp is located in the hanging-wall from which cross-cuts are driven to access the ore at the various lift elevations.

Production comes predominantly from the 1980MASL elevation (120 level) with hanging-wall ramp completed to the 1780MASL elevation (288 level) in 2015, and plans to continue the ramp to depth.

The ramp is approximately 3.5 metres by 3.5 metres and is supported by grouted rebar, as required. There is one main ore zone which is typically 1.5 to 4.0 metres wide and up to 100 metres long on strike.

Drilling is carried out with hand-held jack-leg drills in the narrow sections. Mucking is carried out by 2.0, 3.5 and 4.0-yard LHDs and broken ore is hauled from the mine by 10-tonne highway trucks to the processing plant. Ground support in the stopes, grouted rebar, is installed using stopers or jack-legs.

Development has commenced on the San Cayetano zone, consisting of drifting along the mineralization on the 475 level, minor raising to ascertain grade continuity, and cross cutting in the hanging-wall to build out several drill stations. Mining from the 520 and 475 levels has been conducted by cut and fill.

No significant development has commenced on the Valenciana zones although a minor amount of development was completed re-habilitating a footwall zone.

No development has commenced on the Promontorio zone.

Mineral feed from both the Guanajuato Mine and San Ignacio Mine are processed at the GMC metallurgical plant at the Guanajuato Mine.

The three stage crushing plant is designed to produce ball mill feed that is less than 3/8 inch in size. Run-of-mine ore is passed through a grizzly, into the 1,000 tonne coarse ore bin. Oversize is broken manually or with a backhoe-mounted rock-hammer. The coarse ore is minus 18 inches’ material. From the bin the ore is taken by an apron feeder and over vibrating grizzly to the Pettibone (24” by 36”) primary jaw crusher. The jaw crusher is set to four inches. There is also a second 500 tonne capacity coarse ore bin that feeds a separate, smaller crusher so that aterials can be kept separate if desired.

The jaw crusher product and the undersize from the vibrating grizzly is sent to the Symons (4 ft. standard head) secondary cone crusher, which further reduces the particle size to minus one inch. The secondary crusher product is conveyed to the Allis Chalmers (6 x 16 ft.) vibratory screen, where material, passing through the halfinch mesh, is conveyed to the fine ore bins. Oversize from the screen is fed to the Norberg HP300 tertiary cone crusher, with a design product size of one quarter inch. The tertiary crusher operates in closed circuit with the vibratory screen. The fine ore bin has a total capacity of 2,500 tonnes. Ore from the fine ore bins is conveyed to one of three Denver (7 x 14 ft.) ball mills operating in parallel.

The grinding section is designed to reduce the fine ore to slurry with a particle size of 68% passing 74 microns to liberate the sulphide minerals containing the gold and silver from the waste minerals.

In 2011, the flotation section was greatly improved with the installation of five new fully automated Outotec cells which replaced the old sections of rougher cells. Now the flotation products of these cells are sent according to their quality to cleaning cells or recirculated with scavenger products and cleaner tails for regrinding, after the step of cleaning the concentrate is sent to the concentrate thickener section and filtered to remove excess water, leaving finally an average humidity of 11% and so is sent to the point of sale according to existing contracts. These modifications have been key to improvements in metallurgical recoveries.